The present invention relates recovering a fluid from an underground fluid-bearing formation, wherein a borehole extends from surface to the underground formation, and wherein the fluid is passed to surface through a tube extending through the borehole from the underground formation. In the specification the word "reservoir" will be used to denote an underground fluid-bearing formation. The fluid in the underground formation can be present in the form of a supercritical fluid, a gas, or a mixture of gas and liquid. The fluid can consist of carbon dioxide, natural gas or a mixture of hydrocarbons.
The present invention relates more in particular to an apparatus for compressing a fluid, which apparatus can be arranged in the lower end part of the tube which extends through the borehole.
U.S. Pat. No. 4,684,335 discloses an apparatus for compressing a fluid including a twin rotor screw compressor.
SPE Paper No. 8245, "Field Testing the Turbo-Lift Production System," by H. Petrie and J. W. Erickson, 1979, discloses a liquid poweered downhole liquid pump comprising an open-ended housing and a solid rotor arranged rotatably in the housing. Both the liquid-powered motor and the pump are staged turbines with blades arranged in the annular space between the housing and the solid rotor. The publication does not disclose an apparatus for compressing a fluid including a compressor arranged in a tubular rotor.
It is an object of the apparatus to provide an apparatus which is simpler than the known apparatus and which is furthermore less susceptible to wear at the high fluid flow rates which are encountered as gas is compressed.